Electron discharge devices employing photo-conductive target electrodes



H. G. LUBszYNsKl x-:TAL 2,931,931 ELEcTRoN DISCHARGE DEVICES EMPLOYINGPHOTO-CONDUCTIVE TARGET ELECTRODES v Filed July 2o, 1955 [1o /n 12 gr 1T-1oo-ov 7@ ov `4 April 5, 1960 O -15OM nited States Patent ELECTRONDISCHARGE DEVICES EMPLOYING PHOTO-CONDUCTIVE TARGET ELECTRODES HausGerhard Lubszynski, Waltham St. Lawrence, and

.lohn` Wardley, Hounslow, England, assgnors to Electric & MusicalIndustries Limited, Hayes, Middlesex, England, a company of GreatBritain Application July 20, 1955, Serial No. 523,262

Claims priority, application Great Britain July 27, 1954 6 Claims. (Cl.S13-65) This yinvention relates to electron discharge devices employingphoto-conductive `target electrodes.

Devices `of this kind have been proposed for television and similarpurposes in which a layer of photoconductive material is deposited on atransparent signal electrode formed on a glass window of the envelope ofthe device and the target electrode is arranged to be scanned by acathode ray electron beam so as to generate signals in accordance withthe point-to-point conductivity of the target electrode when an opticalimage is projected thereon. In one form of device the photo-conductivelayer is composed of antimony trisulphide which is deposited in the formof a spongy layer. The glass window of the envelope in such devices isusually circular and the photo-conductive layer is usually depositedover the whole area of the window. In use, however, of such devices itis usually the practice to scan a rectangular raster the orientation ofwhich relatively to the window depends on the disposition of thescanning means, which usually comprise scanning coils surrounding theenvelope of the device. Itis found when such devices are initially usedor tested that if a rectangular raster is scanned on the targetelectrode whilst the latter is illuminated uniformly, the scanned areahas a lower resistance in the dark and a greater sensitivity comparedwith the unscanned area. Furthermore, if a checker board image isprojected on to the target electrode and scanned in a rectangularraster, it is found that the lighter areas of the scanned pattern have alower resistance in the dark and higher sensitivity compared with thedarker areas of the pattern. These elects are permanent and areundesirable since it is obvious that a uniform sensitivity and darkresistance should be present over a raster area of the target electrode.

The object of the present invention is to provide a method of treating aphoto-conductive target electrode with a view to reducing theabove-mentioned effects.

According to the present invention there is provided a method oftreating a photo-conductive target electrode having a spongy layer witha view to increasing the sensitivity and imparting a substantiallypermanent and uniform sensitivity and dark resistance to the targetelectrode which comprises bombarding at least a raster area of thephoto-conductive target electrode with a high beam current whilstsetting up a high voltage across said target electrode and whilst thetarget electrode is illuminated with a uniform illumination. Preferablysubstantially the whole of the area of the target electrode is sotreated particularly since after the initial use of the target electrode re-orientation of the scanned area may occur.

A voltage of the order of 150 volts with respect to the cathode may beapplied to the signal electrode in order to set up a high voltagegradient through the thickness of said target electrode althoughpreferably during treatment of the tube in accordance with the nventionthe voltage of the signal electrode is varied from a low voltage to ahigh voltage of the order of 150 volts and vice versa.

In order that the'said invention may be clearly understood and readilycarried into effect, it will now be more fully described with referenceto the accompanying drawi ing, which illustrates a typical form ofelectron discharge device employing a photo-conductive target electrode.

As shown in the drawing the reference numeral 1 indicates an evacuatedenvelope having a glass end wall 2bn which is formed a transparentsignal electrode 3 having deposited thereon a photo-conductive targetelectrode 4 which may consist of zinc sulphide or cadmium sulphide butpreferably consists of a spongy layer of antimony r-trisulphide having asuperimposed solid layer of antimony trisulphide. Within the envelope 1there'VV is provided an anode 5, the end of which adjacent t0 thephoto-conductive layer 4 is provided with a mesh 6. At the endV of theenvelope 1 remote from the window 2 there is provided an electron guncomprising a cathode 7, a'cathode screen 8 and an apertured electrode 9which limits the cross-sectional area of the electron beam pro' duced bythe cathode. The cathode 7 serves to generate a beam of electrons whichcan be scanned over the photoconductive layer 4 by means of scanningcoils 10. The electron beam is maintained infocus by means of a sole-Anoid `coil 11. The device shown in the drawing is also provided with theusual alignment coils 12.

'In order to avoid the undesirable effects hereinbefore referred to, thefollowing procedure is adopted. The' device is conditioned for operationby connecting the' cathode 7 .to a source of zero voltage, the cathodescreen 8 to a source of voltage which may vary from -100 to zero volts,the limiter electrode 9 to a positive source of 300 volts and the anode5 to a similar source of voltage. ,The signal electrode 3 is connectedto a source of positive voltage whichcan be varied from 0 to 150l voltswhereby a voltage gradient can be set up through the -thickness of thelayer 4. The usual current supply. is applied to the solenoid coil 11and to the alignment coil 12 anda suitable scanning waveform is appliedto the coil 10 so as to enable the whole area of thetargetelectrode tobe bombarded with electrons. The electron beam is also defocussed eitherby varying the normal current supply to the solenoid 11 or by varyingthe voltage of the electrode 8, or both and the beam current is adjustedto a high value. With the device operating in this manner the whole ofthe target area is illuminated with a uniform illumination from a lightsource indicated conventionally at 13, this illumination being of theorder of 15 to 200 foot candles. The voltage applied to the signalelectrode 3 is then slowly raised to 150 volts and down to zero voltsseveral times, say 2 to 6 times, whilst the whole area of the targetelectrode is scanned with said defocused beam. The same process is thenrepeated with the target electrode in the dark. The scanning beam mayemploy a beam current of several microamperes, say up to 10 microampereswhich is high compared with the normal operating beam current of, say,one to two microamperes. Of this latter current however only a fractionthereof reaches the target electrode. Instead of apply ing a voltage tothe signal electrode 3 in order to set up the voltage gradient throughthe layer 4 a similar voltage gradient can be established through thethickness of the photo-conductive layer 4 by applying a suitablenegative voltage to the cathode 7. Such a method is found to result inan overall sensitivity gain of between 3 and 9 decibels over anuntreated target electrode and retention of images is found to besubstantially avoided.

What we claim is:

l. A method of treating the target electrode of an electron dischargedevice which includes a spongy photoconductive layer and prior to normalelectron beam scanning of an optical image of variable light intensityprojected thereon which comprises bombarding at least a rasterarea .'QfJelaldbtarget 4electrode with 'a 'high 'current electron ,streamrwhilst.said .area .is Vvilluminated with ,a uniform intensity illuminationand` whilst a voltage gradient is set up through the lthickness of saidtarget electrede., said illumination, V,current and 'voltage gradientbeing chosen to' increase -the ysensitivity of .said area -and to.impart yaztuxiformsensitivity .and :dark resistance :to said 181263.

2v. A method YVtu' :treating :the target `electrode of an electrondischargedevice which :includes aspongy photoconductive layer and -priorZ,to normal electron beam Scanning of an optical zimage of variablelight yintensity projected thereon :which comprises homharding at leasta1.r aste r1arca oflsaidftarget electrode with a high current electronstream whilst said ,areafis illuminated with a uniformintensitydlluminatiomand whilst va voltage gradient is sc tup `varyingbetween .-a high *,value and a low ttalue A'through the thickness ofsaid target electrode, ysaid illumination, .current Yand-voltagegradient :being chosen -to increase the Sensitivity'of said area .andyto impart a .uniform sensitivity and dark resistance fto lsaid area. 3.A method of tre ating :the 4target -electrode of an electrondischargetdeyice which includes a spongy photo conductive layer andprior -zto normal electron beam scanning pfan Optical image Of-wariablelight intensity projected thereon which comprises `kbombarding :at"least .arasterfarea of said target electrode with a high :currentelectron stream whilst said area -is illuminated with ,a uniformintensity yilluminationand whilst a vvoltage gradientisisetaup throughthethicknessoffsaid area electrode, subsequentlymaintaining:saidtarget-intthe dark andagain bombandingisaid 'areawithahigh-Vcurrentelectron stream, Said illumination, current and ivoltagegradient being chosen `to `increase the sensitivity ,of `lsaid area and-to impart auniform sensitivity-and dark resistanceto s aid area.

`d. A Imethod of treating the `target electrode of an electron dischargedevice which -includesa spongy photoconductive layer and tprior tonormal electron beam scanning of an optical image o f variablelightmintensity proiected :thereon which comprises .bombarding at least.a raster area .of said :target electrode witha l.high current electronstream whilst said area is illuminated with a uniform intensityillumination and whilst a voltage gradient is set up varying between ahigh value and a low value through the thickness of said targetelectrode, subsequently maintaining said target electrode in the darkand again bombarding said area with a high current electron stream, saidillumination, current and voltage gradient being chosen Yto increase thesensitivity of said area and to impart a uniform sensitivity and darkresistance to saidarea.

"5. Arnethod of treating the target electrode of an electron dischargedevice which includes a spongy photoconductive layer and prior to normalelectron beam scanning of an optical image of variable light intensityprojected thereon .which comprises scanning at least `a raster area ofsaid target electrode with a defocussed electron beam of severalmicroamperes whilst said area is illuminated with a uniform intensityillumination greater than the normal operating light intensity of -saiddevice and whilst ,a voltage gradient higherthan that employed in normal1operation o f said device is set up through the thickness of saidtarget electrode, said illuminatiomcurrent :and voltage gradient beingchosen to increase the sensitivity ofsaidfarealand to impart a uniformsensitivity and vdark `resistance to said area.

6,. Anelectron Idischarge deviceV having a target .electrode whichincludes afspongy photo-conductive layer havingan increased sensitivityover a raster area thereof and a uniform sensitivity and dark resistancewithin said area produced, prior to normal electron beam scanning ofanoptical area at variable light intensity, by bombarding at aleast saidraster area of said target electrode with a'high current electron streamwhile said area is illuminated with a .uniform intensity illuminationand while a voltage gradient is set up through the thickness of saidtarget electrode.

References Cited in the tile of this patent UNITED STATES PATENTS2,177,736 Miller, Oct. 31, 193,9 42,741.4837 Forgue May 8, Y19562,837,688 McGee June 3, 1958

